1. Field of the Invention
The present invention relates to a method of manufacturing a multilayer-type chip inductor, and in particular, a multilayer-type chip inductor having a small DC resistance.
2. Description of the Related Art
A method of reducing the DC resistance of a multilayer-type chip inductor involves increasing the cross-sectional area of an internal conductor. In order to increase the cross-sectional area of an internal conductor, the width and the thickness of the internal conductor may be increased. If the width of the internal conductor is increased, however, the inductance decreases. Increasing the cross-sectional area of the internal conductor also may cause various manufacturing problems. Therefore, it is difficult in practice to increase the cross-sectional area of the internal conductor. For this reason, a coil comprising parallel internal conductors has been conceived as a method for decreasing the DC resistance of an inductor.
First, a multilayer-type chip inductor of a first conventional example in which coils are connected in parallel will be described with reference to FIGS. 4 and 5. FIG. 5 is a cross-sectional view of the device shown in FIG. 4.
With reference to both FIGS. 4 and 5, a multilayer-type chip inductor 1 is formed in such a way that green sheets 2a to 2e having electrode films 3a to 3e formed thereon, respectively, are multilayered in two upper and lower stages and sintered together. Further, external electrodes (not shown) are formed on both ends of this sintered body.
The first green sheets 2a to 2e are formed into sheets from an insulating ceramic slurry, such as ferrite or a dielectric. The electrode films 3a to 3e, which become internal conductors, are formed on one surface of the sheets by printing or like technique. Furthermore, in the first green sheets 2b to 2e, via holes 4b to 4e are provided at one end of each of the electrode films 3b to 3e. The upper and lower stages of first green sheets 2a to 2e are multilayered in sequence, causing the electrode films 3a to 3e to conduct in order to form two inductors 5. In parts of the electrode films 3a and 3e, one end of each film is extended to the end of each of the green sheets 2a and 2e so that it connects to and provides conduction with the external electrode (not shown), forming extension electrodes 6a and 6e, respectively.
The multilayer-type chip inductor 1 is obtained in the following way. As shown in FIG. 4, a predetermined number of dummy green sheets 2f on which no electrode film is formed are multilayered in sequence to form a bottom portion of the device. Next, the first green sheets 2a to 2e containing the electrode films 3a to 3e on their respective top faces are multilayered on top of the dummy green sheets 2f. Further, in the same manner, another series of green sheets 2a to 2e are multilayered, and a predetermined number of dummy green sheets 2f are applied. Then, the body is contact-bonded and sintered. Then, external electrodes are formed at both ends (the right side and the left side in FIG. 4) of this sintered body.
Since the first green sheets 2a to 2e shown in FIG. 4 are formed with the electrode films 3a to 3e of a 3/4 turn, respectively, two inductors 5 of 3.5 turns are formed inside the sintered body.
The external electrode on the right side is made to conduct with the extension electrodes 6a and 6a of the inductors 5 and 5, and the external electrode on the left side is made to conduct with the extension electrodes 6e and 6e of the inductors 5 and 5. Therefore, as shown in FIG. 5, the multilayer-type chip inductor 1 is such that the two upper and lower inductors 5 and 5 are connected in parallel.
Next, a multilayer-type chip inductor of a second conventional example comprising a coil of parallel internal conductors will be described with reference to FIGS. 6 and 7. Components in FIGS. 6 and 7 which are the same as those of the above-described first conventional example are given the same reference numerals and a detailed description thereof is omitted.
A multilayer-type chip inductor 11 is formed in such a way that first green sheets 2a to 2e have electrode films 3a to 3e formed thereon, respectively. First green sheets 12a to 12e are similar to the first green sheets 2a to 2e. The green sheets 2a to 2e are alternately arranged (e.g., interleaved) in a multilayered fashion with the green sheets 12a to 12e. These multiple layers are then sintered, and then external electrodes (not shown) are formed at both ends of this sintered body.
The first green sheets 12a to 12e are formed into sheets from an insulating ceramic slurry in the same manner as the first green sheets 2a to 2e, and electrode films 13a to 13e are formed on one surface thereof. Further, in the first green sheets 12b to 12e, via holes 14b to 14e are formed at the ends of the electrode films 13b to 13e, respectively. In the first green sheets 12a to 12d, via holes 17a to 17d are provided at the other ends of the electrode films 13a to 13d, respectively.
The multilayer-type chip inductor 11 is obtained in the following way. As shown in FIG. 6, a predetermined number of dummy green sheets 2f are multilayered in sequence to from a bottom portion. Next, the first green sheets 2a, 12a, 2b, 12b, 2c, 12c, 2d, 12d, 2e, and 12e are multilayered on top of the bottom portion, with each surface having an electrode formed on its top side. Further, a predetermined number of dummy green sheets 2f are applied on top of the body, and then the body is contact-bonded and sintered. Then, external electrodes are formed at both ends (the right side and the left side in FIG. 6) of this sintered body.
Therefore, in the multilayer-type chip inductor 11, an inductor 15 of 3.5 turns which is made to branch into two lines via the respective via holes is formed within the multilayered body. The external electrode on the right side is made to conduct with the extension electrodes 6a and 16a of the inductor 15, and the external electrode on the left side is made to conduct with extension electrodes 6e and 16e of the inductor 15.
However, in the above-described conventional first and second examples, although the DC resistance of the inductor is reduced, the following problems are present. In the first conventional example, because the decrease in inductance is large, the number of windings of the coil must be increased to maintain the inductance at a desired value. In the second conventional example, although the decrease in inductance is small, the number of via holes corresponding to via holes 17a to 17d provided in the first green sheets 12a to 12d and the number of types of first green sheets increases, causing the manufacturing process to become more complex.